Dermatomyositis (DM) exemplifies a group of uncommon but life- and organ-threatening autoimmune syndromes collectively known as idiopathic inflammatory myositis (IIM). Patients with DM suffer debilitating muscle weakness, respiratory impairment, and disfiguring skin rashes. Organ damage in DM is associated with intense inflammatory and immune reactions, both systemic and local; however, the key cellular and molecular investigators of immune dysfunction are unknown. Our recent genomic and proteomic studies reveal striking association between DM disease activity and serum levels of the pro-inflammatory cytokine interleukin-17 (IL-17) and type-I interferon (IFN) regulated chemokines. Further, high levels of IL-17 and IFN-related chemokines are detected in muscle biopsies from both DM patients and from rodents with experimental myositis. These observations lead us to hypothesize that a) increased IL-17 and related molecules will serve as sensitive biomarkers of disease activity and severity in DM and b) dysregulation of the IL-17 axis and the type-I IFN-related chemokines are a key driving force in the immunopathology of DM. We propose to test these hypotheses using genomic, immunohistochemical, and biochemical approaches. First, we will determine the precise anatomic and cellular location of IL-17 in diseased human DM muscle and determine the requirement for and sufficiency of IL-17 in a myositis animal model. Second, we will assess the predictive and diagnostic value of measuring peripheral blood components of the IL-17 axis and IFN-related chemokines in a longitudinal study of Mayo clinic DM patients. Finally, we will explore the therapeutic value of antagonizing the IL-17 axis in myositis. Using injectable anti-IL-17 antibodies, we will determine whether blocking IL-17 function can ameliorate or prevent muscle damage in a mouse model of myositis. Data arising from the proposed experiments will shed new light on the immunopathogenesis of DM, will establish the value of novel biomarkers for DM disease activity assessment, and will quantitate therapeutic potential of IL-17 manipulation in inflammatory myositis.